Cooling systems for operation with a two-phase refrigerant are known from DE 10 2006 005 035 B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US 2010/0251737 A1 and may be used for example to cool food that is stored on board a passenger aircraft and intended to be supplied to the passengers. Typically, the food provided for supplying to the passengers is kept in mobile transport containers. These transport containers are filled and precooled outside the aircraft and after loading into the aircraft are deposited at appropriate locations in the aircraft passenger cabin, for example in the galleys. In order to guarantee that the food remains fresh up to being issued to the passengers, in the region of the transport container locations cooling stations are provided, which are supplied with cooling energy from a central refrigerating device and release this cooling energy to the transport containers, in which the food is stored.
In the cooling systems known from DE 10 2006 005 035B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US 2010/0251737 A1 the phase transitions of the refrigerant flowing through the circuit that occur during operation of the system allow the latent heat consumption that then occurs to be utilized for cooling purposes. The refrigerant mass flow needed to provide a desired cooling capacity is therefore markedly lower than for example in a liquid cooling system, in which a one-phase liquid refrigerant is used. Consequently, the cooling systems described in DE 10 2006 005 035 B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US 2010/0251737 A1 may have lower tubing cross sections than a liquid cooling system with a comparable cooling capacity and hence have the advantages of a lower installation volume and a lower weight. What is more, the reduction of the refrigerant mass flow makes it possible to reduce the conveying capacity needed to convey the refrigerant through the cooling circuit of the cooling system. This leads to an increased efficiency of the system because less energy is needed to operate a corresponding conveying device, such as for example a pump, and moreover less additional heat generated by the conveying device during operation of the conveying device has to be removed from the cooling system.
A cooling system installed on board an aircraft must be capable for operation under various environmental conditions. For example, the cooling system must be capable to be operated at very high, but also at very low ambient temperatures. To maintain an undesired heat introduction into the cooling system at high ambient temperatures as low as possible, the tubing of the cooling system, in particular the tubing of a cooling circuit of the cooling system is insulated. The insulation of the tubing of the cooling system, however, in particular during longer immobilization times of the aircraft or during longer loading cycles with open cargo doors at low ambient temperatures usually is not sufficient to prevent the temperature of the tubing from falling below a dew point of the two-phase refrigerant. This may result in an undesired condensation of the two-phase refrigerant at the cold walls of the cooling circuit tubing which is further promoted by the high tubing lengths typical in many aircraft cooling systems.
The liquefied refrigerant may accumulate in the tubing of the cooling circuit and thus may no longer be available for circulation through the cooling circuit. This may cause failure of the cooling system. Nevertheless, a reliable operation of the cooling system at low ambient temperatures can be achieved by appropriately overdesigning the cooling circuit and in particular the amount of two-phase refrigerant circulating through the cooling circuit such that condensation of a part of the two-phase refrigerant at the cold walls of the cooling circuit tubing can be compensated for by excess gaseous refrigerant still present in the cooling circuit. To keep the system's structural and operational complexity as well as the system weight as low as possible and also for safety reasons it is, however, desirable, to employ as little refrigerant as possible.